1. Field of the Invention
Embodiments of the invention relate to the field of device fabrication. More particularly, the present disclosure relates to a scanning method for ion implantation utilizing a shadow mask.
2. Discussion of Related Art
Ion implantation is a standard technique for introducing conductivity-altering impurities into substrates. A precise doping profile in a substrate and associated thin film structure is critical for proper device performance. Generally, a desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the substrate. The energetic ions in the beam penetrate into the bulk of the substrate material and are embedded into the crystalline lattice of the substrate material to form a region of desired conductivity.
Such an ion implanter may be used to form solar cells. Solar cells are typically manufactured using the same processes used for other semiconductor devices, often using silicon as the substrate material. A semiconductor solar cell has an in-built electric field that separates the charge carriers generated through the absorption of photons in the semiconductor material. This electric-field is typically created through the formation of a p-n junction (diode) which is created by differential doping of the semiconductor material. Doping a part of the semiconductor substrate (e.g. surface region) with impurities of opposite polarity forms a p-n junction that may be used as a photovoltaic device converting light into electricity. These solar cells provide pollution-free, equal-access energy using a recurring natural resource. Due to environmental concerns and rising energy costs, solar cells are becoming more globally important. Reducing cost to manufacture or increasing production capability of these high-performance solar cells or other efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
Solar cells may require doping to improve efficiency. This may be seen in FIG. 1 which is a cross-sectional view of a selective emitter solar cell. It may increase efficiency of a solar cell to dope the emitter 200 and provide additional dopant to the regions 201 under the contacts 202. More heavily doping the regions 201 improves conductivity and having less doping between the contacts 202 improves charge collection. The contacts 202 may only be spaced approximately 2-3 mm apart. The regions 201 may only be approximately 50-300 μm across.
FIG. 2 is a cross-sectional view of an interdigitated back contact (IBC) solar cell. In the IBC solar cell, the junction is on the back of the solar cell. The doping pattern is alternating p-type and n-type dopant regions in this particular embodiment. The p+ emitter 203 and the n+ back surface field 204 may be doped. This doping may enable the junction in the IBC solar cell to function or have increased efficiency.
In the past, solar cells have been doped using a dopant-containing glass or a paste that is heated to diffuse dopants into the solar cell. This does not allow precise doping of the various regions of the solar cell and, if voids, air bubbles, or contaminants are present, non-uniform doping may occur. Solar cells could benefit from ion implantation because ion implantation allows precise doping of the solar cell. Ion implantation of solar cells, however, may require a certain pattern of dopants or that only certain regions of the solar cell substrate are implanted with ions. Previously, implantation of only certain regions of a substrate has been accomplished using photoresist and ion implantation. However, use of photoresist, adds an extra cost to solar cell production because extra process steps are involved. Other hard masks on the solar cell surface are expensive and likewise require extra process steps. There are advantages in implanting small, regions of solar cells and having a lower sheet resistance between implanted regions to improve series resistance. Both may be accomplished through the use of ion implantation. Accordingly, there is a need in the art for an improved method of implanting through a shadow mask and, more particularly, a scanning method for ion implantation that uses a shadow mask with solar cell fabrication.